there are lots of foams that are machinable and are open for water, also there is a foam glass that is normally closed cell but is available reticulated, also know as fart rock, used as a fire proof insulation. depends on what you are doing.http://www.crestfoam.com/bull101new.html

Porous Materials:

A number of refractory materials are porous and, at least to some extent, machinable. Contact some of the industrial refractory supply companies (or installers) in your area. You will find more of them than you think. Probably can get samples. Both insulating firebrick and castables come to mind. We (employer) used to use them before I retired.

A number of metals are available in sintered form and with varying porosities.

If we knew your application we could better recommend something.

The pottery industry uses plaster of Paris as molds to suck the water from casting slip - everything from toilets to pots.

I would suggest nickel foam, but I'm not sure what the retail price would be, how large are these molds going to be? On the other hand, there are some refractories as mentioned, we use a type of alumina that comes in various porosities, from vesuvius, it is used widely as a filter in casting operations, so it is disposable. It is easily machined too.

The molds could be 8" X 24" X 24" larger or smaller. They sit in water so aluminum corrodes fast. Currently using a plastic but not porous. It has to have a bunch of holes drill in it which isn't practical.

How about porous plastic, like one of the polyethylenes (LDPE, HDPE, or UHMW)? We fabricate something similar at work for propietary R&D projects, but there are probably fabricators out there that can make something for you. Machinability might be a problem, especially for the lighter grades of PE.

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